Adviser: 葉慶隆 教授Reporter:GI2 16 黃凱聖

創辦人為昇陽電腦的 Bill Joy

昇陽電腦認為，只有互通才能真正發揮出P2P 的優勢，就好像即時通訊 IM(Instant Messaging) ，能互連的人越多，越有價值。所以 Sun 決定出面發佈一個平台，使所有P2P 系統都能連接起來，為了達到互通性 發表了一款概念最簡單的系統 如果成功，這就會是今後大家構架信息系統的基本模塊

昇陽電腦認為 JXTA 計畫在點對點（ P2P ）運算方面的研發成果將對搜尋、分享、與儲存資訊有所助益，而這些根本元素都和發揮網際網路最大效益息息相關。

最新版為 JXTA 2.5

名稱起源於 Juxtapose( 並列之意 ) 漂亮而優美地描繪未來完整的 P2P 世界

The JXTA protocols are a set of protocols that have been specifically designed for ad hoc, pervasive, and multi-hop peer-to-peer (P2P) network computing.

What this all means is that JXTA is a framework with a set of standards that support peer-to-peer applications.

JXTA is made up of three distinct layers. The first is the platform.

The platform contains core functionality used by services, which are the second layer.

Services provide access to the JXTA protocols.

Finally, there are applications that use services to access the JXTA network and utilities.

Operating system independence.

Language independence.

Providing services and infrastructure for P2P applications.

There are also conceptual goals:• Use groups to organize peers and to give context to

services and applications.

• Groups use authentication and credentials to control access and/or enable security at the group level.

• Distribute information about peers and network resources throughout the network.

• Queries are distributed throughout the system.

• Provide an infrastructure for routing and communications between peers. Communication with peers behind firewalls and other barriers is a key part of this goal.

• Provide mechanisms to allow peers to monitor each other and resources.

XML is the basis for most of the protocol in JXTA.

The key reasons are its ability to be read by many languages and its ability to be validated.

There is a downside to using XML.

XML is simply not a compact way to express data.

Messages written in XML will be much larger than a binary equivalent.

There are techniques that can be used, such as replacing tags with binary tokens or compacting data, but none of these are currently employed in JXTA because there are no widely accepted standards at this time.

Consequently, the core JXTA developers have created a simple binary message transport and have used terse language and acronyms for tag names.

Peer• A peer is a virtual communications point. You

can have multiple peers on a computer or device.

• It is also possible to have multiple peers on a single device, not necessarily an ideal situation but good for debugging.

Peer Group• A peer group is a way to group peers and to

advertise specific services that are available to group members. You can create groups, join them, and of course resign from a group. There is also the ability to renew a membership in a group.

Peer Group• A group may need to limit membership for

various reasons, such as secure communications between members, privacy, or there may need to be certain information that a user must supply before joining a group.

• There is an authentication protocol specifically designed to collect information and allow the group to determine if the information meets the requirements for membership.

Peer Group• Membership to a peer group can take several

forms. The two key models are local and remote membership services.

• A local membership service runs entirely on the peer that is applying for membership. All resources and ability to validate a user reside also on the same peer. So local membership services allow you to join without connecting to any other peer.

Peer Group• Remote membership requires accessing one or

more peers in the group that you are joining. The idea is that peers that already belong to the group either have access to resources for validating a new member. Similarly, the set of peers in the group could query their users with the new member's application to see if the members are willing to accept the new member.

Endpoint• The endpoint is the basic addressing method used

by JXTA applications to communicate with each other. An endpoint is an address of a peer that implements a specific protocol of communication.

• A simple example of an endpoint is an IP address and port. By using these values, a stream could be opened to communicate to the target peer. However, JXTA places a layer on top of streams called pipes . Instead of connecting a stream to an address (represented by the endpoint), you connect a pipe to the endpoint.

Pipe• A pipe is a virtual connection between peers.

Normally, we think of peer-to-peer communications as a single connection, but this is not always possible. The problem is that many peers cannot connect directly because of firewalls or other barriers. Pipes are intended as a layer over multiple communication protocols and to support relayed communications via gateway peers.

Advertisement• An advertisement is an XML document that

describes a JXTA message, peer, peer group, or service. Advertisements follow standards for encoding, tags, and content. The advertisement is used to exchange information about what is available in the JXTA network.

Messaging in JXTA is done in two different ways. First is the standard way that would be expected with XML.

The second type of message is a very economical binary message.• Compactness• Many messages are already binary• Encryption

A rendezvous is a peer that processes queries from other peers. The rendezvous can also delegate queries to other peers, which must also be a rendezvous.

A key purpose of rendezvous is to facilitate searching of advertisements beyond a peer's local network.

A router in JXTA is any peer that supports the peer endpoint protocol. Not all peers need to implement the protocol because, like traditional network routers, you only need a few to support a large network. JXTA routers are very similar to a traditional router. The primary difference is that a P2P network is less stable and includes many addresses that are not static.

A gateway is a peer that acts as a communications relay. Don't confuse gateways with rendezvous. A gateway is used to relay messages between peers, not requests.

Gateways are like radio repeaters or a middleman between peers used to relay messages.

Peers may also seem to disappear and reappear. We also have to consider wireless devices that are usually only on-line for very short periods. In these cases, the peer may even seem to pop up in a different city in a completely different network topology!

Because of the possibility of such changes, it is very important to be able to invalidate a route and reroute connections.

JXTA protocols are used to help peers discover each other, interact, and manage P2P applications.

The protocols hide a lot of detail, which makes writing JXTA applications much easier than developing a P2P applications from scratch.

Peer Discovery Protocol (PDP)• Allows a peer to discover other peer

advertisements (peer, group, service, or pipe). The discovery protocol is the searching mechanism used to locate information. The protocol can find peers, peer groups, and all other published advertisements. The advertisements are mapped to peers, groups, and other objects, such as pipes.

Peer Resolver Protocol (PRP)• Allows a peer to send a search query to another

peer. The resolver protocol is a basic communications protocol that follows a request/response format. To use the protocol, you supply a peer to query and a request message containing XML that would be understood by the targeted peer. The result is a response message.

Peer Information Protocol (PIP)• Allows a peer to learn about the status of

another peer. The information protocol is used partially like ping and partially to obtain basic information about a peer's status.

Peer Membership Protocol (PMP)• Allows a peer to join or leave a peer group. The

protocol also supports the authentication and authorization of peers into peer groups.

Pipe Binding Protocol (PBP)• It is used to create the physical pipe endpoint to a

physical peer. It is used to create a communications path between one or more peers.

Rendezvous Protocol (RVP)• The Rendezvous Protocol is responsible for

propagating messages within JXTA groups. The Rendezvous Protocol defines a base protocol for peers to send and receive messages within the group of peers and to control how messages are propagated.

Peer Endpoint Protocol (PEP)• It is used to create routes to route messages to

another peer. The protocol uses gateways between peers to create a path that consists of one or more of the pipe protocols suitable for creating a pipe. The pipe binding protocol uses the list of peers to create the routes between peers.

We can also describe these protocols in terms of what they provide to a JXTA application.• Peer Discovery—Resource search• Peer Resolver—Generic query service• Peer Information—Monitoring• Peer Membership—Security• Pipe Binding—Addressable messaging • Rendezvous—Propagation messaging• Peer Endpoint—Routing